On Thu, 8 Aug 2013, Christoph Hellwig wrote:
> On Thu, Aug 08, 2013 at 12:43:26AM -0700, Kent Overstreet wrote:
> > I seem to recall from looking at the logs that you just removed them
> > because all the old users could be and were converted to something
> > saner, for what they were doing (using them as completions, I want to
> > say?)
>
> We explicitly converted them away so that we could kill it. This was
> a joint project with Thomas.
>
> > Bcache isn't using the rw sem as a completion though, it really is a
> > read/write lock that protects a specific data structure, and where
> > we're taking a read lock for the duration of write IOs - and since bios
> > are asynchronous, that's why we need the non_owner() bit.
>
> Part of this commit was to make the rw_semaphore behaviour similar to
> plain mutex, that is making sure there is exactly one owner and not
> different processes locking/unlocking it. This is useful for PI (that's
> why the rt folks care), lock debugging and kinds of other use cases.
Right. We had to implement an anon_rw_semaphore version, which caused
more headache than it was worth the trouble.
The solution for one of the non owner use cases was something like the
below:
read_lock(x->lock);
atomic_inc(x->io_active);
launch_io();
read_unlock(x->lock);
On the writer side:
write_lock(x->lock);
while (atomic_read(x->io_active) {
write_unlock(x->lock);
wait_event(x->wait_for_io, !atomic_read(x->io_active));
write_lock(x->io_active);
}
....
On the io side:
complete_io()
if (atomic_dec_and_test(x->io_active) &&
waitqueue_active(x->wait_for_io))
wake_up(x->wait_for_io);
That would fit into the bcache use case afacit.
Thanks,
tglx
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